Abstract This thesis deals with infrared spectroscopy investigations on the linear DCCI molecule. The high resolution spectra between 200–5200 cm-1 were measured with the Fourier transform spectrometer at the University of Oulu. The spectra were analyzed taking into account various types of resonances between rovibrational energy levels. As a result, a set of molecular constants and resonance parameters describing the rotational and vibrational energy states of the molecule were obtained. From the resulting molecular constants, together with previous results from literature for HCCI, the structure of monoiodoacetylene was calculated. In addition, eight harmonic force constants with estimated uncertainties for monoiodoacetylene were determined.